In the past, it has been observed that microwave heating (i.e., temperature rise of a load) is related to its thermal properties, its dielectric properties (at the frequency of operation), and the extent that microwave energy is coupled to the load, microwave propagation into the load is affected by reflections due to impedance mismatches, and diffraction due to geometrical curvature of such mismatches. All the mentioned parameters are of use to control heating rate.
Controlling microwave absorption by manipulating material composition is possible to some extent, but it is difficult and has limitations, which are sometimes severe. The thermal and dielectric properties are functions of the material composition, and are often interdependent. Other requirements may constrain the material composition; therefore, controlling microwave absorption by manipulating material composition has limited utility.
Controlling microwave absorption with frequency is impractical because the frequencies for industrial, scientific, and medical uses are mandated to 915 and 2450 MHz by law.
The microwave applicator design is the primary method available in practice to control heating. To have high efficiency, it is necessary to minimize energy reflected back to the microwave generator.
Coupling is the degree to which energy is delivered from the feed structure to the cavity compared to the energy reflected back from the cavity to the feed structure. A "matched" condition where the delivered energy balances the reflected energy and no power is returned to the generator is termed critical coupling. This principle can also be applied to the free space field-load interface within the cavity. Critical coupling here allows the greatest microwave field strength in the material to be created, thereby allowing the highest microwave heating rate possible.
The coupling factor is normally defined by an equivalent lumped circuit model of cavity behavior, as the quotient between the feed impedance as seen from the cavity and the combined cavity and load impedance as seen from the feed. The former can be controlled by mechanical changes of the coupling port. The latter is determined by the mode type in the cavity, and by the geometry and dielectric properties of the load. If the load has a low (relative) loss factor .epsilon." the latter generally becomes large. When these impedances are equal, critical coupling occurs and the coupling factor is 1. Thus, manipulation of the coupling is the most efficient way of optimizing the overall heating efficiency with any load in the cavity. Furthermore, it can easily be measured by replacing the magnetron generator by microwave instrumentation incorporating a variable frequency generator and means such as directional detector for measuring the reflected power as a function of the input frequency. Both the resonance bandwidth and the coupling factor can then be easily quantified, and observed continuously on the instrument display during changes of the coupling and other parameters. However, any influence by the dynamic temperature dependence of the dielectric properties of the load cannot be studied directly, since the instrumentation generator power is only in the order of milliwatts.
Of course, varying the generator power output provides an additional means for controlling the load power. Industrial size variable power generators are commercially available and quite reliable. It is therefore possible to set the optimum load power at or near the favorable critical coupling condition.
Most resonant cavities described in the literature are of fixed geometry. If the geometry and dielectric properties of the load material vary substantially, and the same installation is to be used under such strongly variable conditions, the option of changing some cavity dimensions becomes interesting. In particular, relatively low-loss loads create a relatively high quality factor (Q value) which in turn results in a narrow bandwidth of the cavity resonance, which in turn may make it necessary to change the cavity dimensions since the frequency of operation is fixed. This is called cavity tuning, and is not to be confused with the feed port changes, which is called impedance tuning. The former changes the resonant frequency, and the latter changes the coupling factor. They are essentially independent.
In the literature, most tunable resonant cavities for microwave heating have limited tuning ability. Various methods have been employed for tuning including a single moveable end wall to change the cavity length, movable coaxial antennas, stubs and tubes. Most tunable resonant cavities have been designed for batch processing, usually for small quantities of material. Correspondingly, most tunable resonant cavity systems are low power, i.e. less than 10 kW.
U.S. Pat. No. 4,714,812 discloses a tunable cavity having one movable wall that overcomes many of the shortcomings of other designs for high power, continuous process, industrial use. The cavity of this patent has a movable piston at one end of a cylindrical cavity through which product flows in a vertically arranged dielectric tube while being subjected to microwave energy. At least two of these cavities are used together in series to form a working system. The first cavity heats the material and the second cavity monitors the material condition. Power is transmitted to the first cavity through a coaxial transmission line. The maximum power delivered to the first cavity is 5 kW. The movable piston in the first cavity is positioned to maximize the forward power into and minimize the reflected power out of the first cavity. The second cavity is used to determine the material condition by adjusting the cavity length to provide significant reflected power, and then using the reflected power to indicate the state or value of the material property of interest, for instance the moisture. A controller adjusts the power applied to the first cavity based on the monitored value of the second cavity. Liberated moisture is removed from the system in a third unit where crossflow air is passed through perforations formed in the vertical plastic tube carrying the material.
However, a need continues to exist for microwave applicators achieving higher heating rates than existing continuous, industrial microwave applicators and which overcome the shortcomings of such existing applicators.